Assault on the Control Room Shortcut
From Halopedia, the Halo wiki
The Assault on the Control Room Shortcut is a trick that can be performed on the Halo: Combat Evolved level Assault on the Control Room to get to the bottom of the first canyon without spawning any enemies.
The shortcut requires very good timing, so it may take some practice before you can get it. There are two ways to do the trick:
First, go on to the first bridge, and make sure you have a plasma grenade. Try not to wake up any Grunts, as they will be annoying and dangerous. Throw a plasma grenade on the ground, right next to a Shade. Right when the grenade explodes, quickly press X on Xbox (default controls) or E in Halo: Combat Evolved (PC port) to get in the Shade. If you're lucky, the shade will right itself and you will fall safely to the ground.
Another way that requires skill, practice, luck and co-op is to jump. This cannot be done on Halo PC. First, when you get to the bridge with the sleeping grunts, jump off next to the Shade on the left side, and you will fall on a thin ledge. Stay close to the wall to do this. If you accidentally wake up any Grunts, the Elites will shoot at you. Since you lose shields when you fall, you can die. Run down the ledge to the lowest part, then a little further. Have your partner commit suicide, then fall down. You must be close to the wall or this will not work. You will fall past a ledge, where your partner should respawn. You will respawn next to him. Run along the ledge, taking care not to fall as it is slippery. Run over to the part that is directly above the large rocks. Drop down onto them, and you will survive.
At the bottom there are no enemies, and all the usable vehicles that the enemy usually steals can be used. The player can continue on with the level until the end with no enemies present, although sometimes some doors may become locked and certain glitches need to be used to bypass them.